Add rubocop to CI

.github/workflows/rubocop.yml

@@ -0,0 +1,20 @@
+name: Lint Ruby
+
+on:
+  push:
+    branches:
+    - master
+  pull_request:
+
+jobs:
+  rubocop:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: 2.7
+    - name: Lint Ruby code with RuboCop
+      run: |
+        bundle install --jobs 4 --retry 3
+        bundle exec rubocop

.rubocop.yml

@@ -14,7 +14,7 @@ AllCops:
     - 'gemfiles/vendor/**/*'
     - 'clowne.gemspec'
   DisplayCopNames: true
-  TargetRubyVersion: 2.5
+  TargetRubyVersion: 2.7
 
 Markdown:
   WarnInvalid: true